1. Field of the Invention
The invention relates to a device for operating a high pressure discharge lamp. The invention relates especially to a device for operating a high pressure discharge lamp which comprises an ultra-high pressure discharge lamp of the AC operating type in which an arc tube is filled with greater than or equal to 0.15 mg/mm3 mercury, in which the mercury vapor pressure during operation is greater than or equal to 110 atm, and which is advantageously used as a projection light source of a projection device of the projection type or the like and a device for operating this ultra-high pressure discharge lamp.
2. Description of the Prior Art
In a projector device of the projection type there is a demand for illumination of images onto a rectangular screen in a uniform manner and with adequate color rendition. Therefore, metal halide lamps filled with mercury and a metal halide have been used as the light source. Furthermore, recently smaller and smaller metal halide lamps, and more and more often point light sources, have been produced and lamps with extremely small distances between the electrodes are being used in practice.
Against this background, recently, instead of metal halide lamps, high pressure discharge lamps with an extremely high mercury vapor pressure, for example, with a pressure of at least 200 bar (197 atm), have been used. Here, the broadening of the arc is suppressed by increased mercury vapor pressure, the arc is compressed and a great increase of light intensity is the goal.
Recently, there has been a focus on smaller and smaller projector devices. In the discharge lamp for the above described projector device, on the one hand, there has been a demand for a high light intensity and high degree of maintenance of illuminance. On the other hand, according to the reduction in size of the projector device, there is a demand for smaller and smaller discharge lamps. Therefore, smaller and smaller devices, and smaller and smaller power sources are being used. A reduction in the voltage during starting, in other words, a property to facilitate starting, is expected.
For the above described lamp, for example, an ultra-high pressure discharge lamp is used in which, in a silica glass arc tube, there is a pair of electrodes with a distance of less than or equal to 2 mm opposite and in which this arc tube is filled with greater than or equal to 0.15 mg/mm3 mercury, rare gas and halogen in the range from 1×10−6 μmole/mm3 to 1×10−2 μmole/mm3 (for example, see patent 1 and patent 2 listed below). Furthermore, such a discharge lamp and the operating device for it are disclosed, for example, in patent 3 listed below.                (Patent 1): JP-A HEI 2-148561 (U.S. Pat. No. 5,109,181)        (Patent 2): Japanese patent 2980882 (U.S. Pat. No. 6,271,628)        (Patent 3): JP-A 2001-312997 (U.S. Pat. No. 6,545,430 B2).        
In the high pressure discharge lamp disclosed in patent 3, at a mercury vapor pressure within the tube of 15 MPa to 35 MPa in rated operation, the arc tube is filled with a halogen material in the range from 1×10−6 μmole/mm3 to 1×10+2 μmol/mm3. Placing a pair of electrodes within the arc tube and placing a projection part in the vicinity of the middle of the electrode tip area suppress formation of the arc jump phenomenon. An AC voltage is applied by an operating device which comprised of a DC/DC converter, a DC/AC inverter and a high voltage generation device, between the above described pair of electrodes, and thus, operation is carried out.
In such an ultra-high pressure discharge lamp, the phenomenon occurs that projections are formed and grow on the tips of the opposed tungsten electrodes within the arc tube in the course of operation. These projections form and grow dramatically if especially AC operation is carried out with a distance between the electrodes of less than or equal to 1.5 mm, an amount of mercury of at least 0.15 mg/mm3 and an amount of a halogen, such as bromine or the like, from 10−6 μmol/mm3 to 10−2 μmol/mm3.
The phenomenon that projections are formed on the electrode tips is not always clear. However, the following can be assumed.
In such a discharge lamp, the arc tube is filled with halogen gas. The main objective is to prevent devitrification of the arc tube. The halogen gas also yields the so-called halogen cycle. The tungsten which, during lamp operation, is vaporized from the area with a high temperature in the vicinity of the electrode tip reacts with the halogen and the remaining oxygen which are present within the arc tube, and forms a tungsten compound, such as WBr, WBr2, WO, WO2, WO2Br, WO2Br2 or the like if, for example, the halogen is Br. These compounds decompose in the area with a high temperature in the gaseous phase in the vicinity of the electrode tip and form tungsten atoms or cations. The tungsten atoms are transported by thermal diffusion (diffusion of the tungsten atoms from the high temperature area in the gaseous phase, i.e., from the arc, in the direction to the low temperature area, i.e., the vicinity of the electrode tip) and in the arc, become cations and during half-cycles when an electrode operates as the cathode are attracted by the electrical field in the direction to the electrode (drift). It can be imagined that, in this way, the density of the tungsten vapor in the gaseous phase in the vicinity of the electrode tip is increased and tungsten is precipitated on the electrode tip, by which projections are formed.
These projections have the effect that they can prevent the arc jump in the sense of fixing the arc hot spot on these projections if they do not grow. But if in the course of continued operation of the lamp the projections grow, the disadvantages arise that the distance between the electrodes is reduced, that the position of the arc radiance spot is changed, that the light intensity is reduced and similar disadvantages.
In patent 3, it is shown that by the formation of the above described projection part the lamp voltage fluctuates (decreases). Furthermore, it is disclosed here that, in the case of a change of the lamp voltage (of the distance between the electrodes) by the formation of the projection part, by controlling the amount of current flowing between the two electrodes, and by switching the first operating frequency to a second frequency, the fluctuation of the lamp voltage is corrected by the formation of the projection part.
For example, with respect to the amount of current flowing between the two above described electrodes the following is shown:                If the lamp voltage (distance between the electrodes) becomes smaller than the normal value, the length of the projection part is reduced by increasing the discharge arc current which flows between the two electrodes, by which the lamp voltage rises. If the lamp voltage (the distance between the electrodes) becomes greater than the normal value, the length of the projection part is increased by the reduction of the discharge arc current.        
Based on these ideas, in the operating device described in patent 3, a higher discharge arc current is allowed to flow if the determined lamp voltage is less than the reference voltage. Furthermore, the above described DC/DC converter is controlled with feedback here such that the discharge arc current is reduced when the lamp voltage is higher than the reference voltage. Thus, the fluctuation of the lamp voltage is suppressed.
It can be imagined that control of the change of the distance between the electrodes by the operating frequency, which control is described in the above described patent 3, can be effective in certain cases. However, it was found that the growth of the projections often cannot be advantageously controlled.
In patent 3, the value of the increase or decrease of the determined value of the lamp voltage is determined with respect to the reference voltage (initial value of the lamp voltage during aging operation) and the fluctuation of the distance between the electrodes with feedback is controlled by switching of the two values 150 Hz and 800 Hz.
However, as a result of research by the present inventors, it was found that the growth of projections cannot always be advantageously controlled by this type of control. This publication especially discloses a process for two-stage alteration of the operation frequency. Since in this control the lamp voltage changes rapidly, as can be imagined, stable maintenance of the lamp voltage and of the distance between the electrodes becomes difficult, as can be imagined.